The present invention relates to a nut and a stud connected as a single assembly.
Separate nuts and studs are commonly used for assembly of many components and have certain distinct advantages over the use of bolts, such as: better torque-tension control and the use of the headless stud for holding or positioning during assembly. The disadvantages lie in difficulty in driving studs, the labor cost of assembly (two separate items as compared to a single piece item such as a bolt) and the inventory requirements of two pieces instead of one.
In the present invention a nut and stud assembly is provided and can be used in place of a single piece bolt or separate nut and stud. The operator can handle the nut and stud assembly as a bolt, thus offsetting the labor requirements of a two piece installation.
The stud portion of the assembly has one end conventionally threaded as a standard bolt and the opposite end threaded to a selected length for nut engagement and rundown. The nut end portion of the stud incorporates an area of controlled interference, precisely displaced relative to the normal thread configuration to provide interference to preposition and hold the nut. The nut is then preinstalled to this interference area and a nut and stud assembly is produced that can be handled in use as a single piece item similarly to a bolt. In this way the nut and stud assembly can be inventoried by the user as a single item.
The interference provided to retain the nut to the stud has a dual function. The interference provides a controlled magnitude of torque which must be overcome or exceeded to allow the nut to pass over the interference area and to continue over the remaining thread area. The magnitude of torque to overcome this interference is selected to exceed the torque resistance in the tapped hole, such as that caused by burrs, poorly tapped threads (from worn taps) or the increase of frictional build-up as multiple threads are engaged. This ensures that the stud will be threaded into the tapped hole prior to application of loading on the nut to provide clamp up and hence further inhibits damage to the threads in the tapped hole from nut loading.
The nut and stud assembly will continue to move the stud into the tapped hole until a preselected stop is reached. As will be seen the stop can be defined by the position at which the stud bottoms out in the tapped hole or a position on the stud at which a stop, such as additional interference, has been located for resisiting further movement of the stud into the tapped hole. The increased torque on the nut at this point overcomes the positioned thread interference with which the nut is engaged. Now the nut and stud assembly at this juncture operates essentially as the separate nut and stud, with the nut being free running on the stud and creating a clamp up load on workpieces as a two piece fastener. At the same time, upon the requirement for disassembly of the secured workpieces, the nut, upon reverse rotation, will again engage the interference section with the residual interferences resisting removal of the nut but assisting in removal of the nut and stud assembly as a unit thus facilitating disassembly of the workpieces.
Preferably the nut utilized in the nut and stud assembly can be of a laminated structure. In this regard such laminated nuts utilize laminations of hex shaped, hardened coned-disc springs, having a central opening tapped to match threads on an associated bolt. Such nuts have stacked aligned multiple discs retained by a steel outer cage having a hex shape mating with that of the nut. Examples of such laminated nut structures are shown and discussed in U.S. Pat. No. 4,383,787, issued to Reynolds on May 17, 1983.
The laminated nut can be used with a hardened stud and itself can be hardened to a higher hardness facilitating movement through the stud interference without damage to the nut threads. In this regard the laminated nut has an advantage over a conventional nut which is limited in the hardness level attainable by stresses that could result in cracking of the nut because of its brittleness.
The laminated nut construction also has the advantage of providing a desired load retention characteristic. In this regard, the laminated nut is free running on the bolt until seated. After initial clamping of the workpieces, a locking force between the nut and bolt results from the application of additional torque and rotation. The additional torque compresses and partially flattens the conical spring discs whereby a mechanical interference occurs between the thread flanks of the nut and bolt. The resultant thread interference and retained spring load between thread flanks resists unloading that might otherwise occur as a result of tensile or vibrational loads. The retained spring load also is advantageous where the workpieces are subject to thermal and pressure cycling such as found in engine block and cylinder head assemblies. Thus, in a preferred form of the present invention, the laminated nut and stud assembly has the advantages of a laminated nut while providing the advantages of a bolt, i.e. ease of operator installation and reduction in parts inventory of the conventional two piece nut and stud combinations.
Therefore it is an object of the present invention to provide a new and unique nut and stud assembly in which the nut is threadably secured to one end of the stud and its travel on the stud held off until a preselected torque is reached.
It is another object of the present invention to provide a new and unique nut and stud assembly which has the advantages of a bolt in handling and inventory and the advantages of a separate nut and stud combination in fastening workpieces together.
It is still another object of the present invention to provide a new and unique nut and stud assembly in which the nut is of a laminated construction.
It is a general object of the present invention to provide a new and unique nut and stud assembly.